AWUS01 KWNH 222025
FFGMPD
TXZ000-230200-

Mesoscale Precipitation Discussion 0963
NWS Weather Prediction Center College Park MD
425 PM EDT Tue Aug 22 2023

Areas affected...South Texas...

Concerning...Heavy rainfall...Flash flooding likely

Valid 222030Z - 230200Z

SUMMARY...Tropical Depression Harold. Training Band near Corpus
Christi likely to continue localized flash flooding risk over the
next few hours, as main core pushes into Mexico.

DISCUSSION...Recent visible imagery shows the sheared Central
Dense Overcast continues to erode rapidly as the mid-level
shortwave and northeasterly flow elongates the system from the ESE
toward the WNW from the center in E Jim Hogg county toward the
nose of the strong WAA/low to mid-level convergence center across
Webb county. As such sfc to boundary layer convergence has
sharpened and maximized with two arcs of increased convection and
extreme moisture flux, given 2.6-2.7" of total PWAT transported
along 40-50kts of 850-700mb flow with 30-45 degrees of wind
shift/change to continue isentropic ascent into the Rio Grande
Valley. Given the angle of ascent in the shield is a bit less
vertical, rain rates/totals across the NW Quadrant have been
.5-1"/hr per backyard observations, with embedded cores
(particularly near the intersection the inner core/band near the
Duval/Webb county line) with quick .5-1" bursts in 10-15minutes,
resulting in a broad area of 1-3" expected across western portions
of South Texas in the next few hours. Spots of flash flooding is
possible.

However, it is the upstream bands that draw more attention and
concern. As the circulation shears/elongates, the inner confluence
band is orienting favorably to the mean storm motion resulting in
a nearly stationary band across S McMullen county back toward
Nueces and S San Patrico counties, where heavy rainfall has
already occurred resulting in localized flash flooding conditions.
At this time, a few more hours of this narrow band appear to
remain stationary likely compounding flash flooding in a very
narrow band. Due to proximity to the very warm western Gulf of
Mexico, ample unstable air remains in place upstream, and with
strong 40-50kts of 850mb with weak but sufficient confluence
supports propagation vectors back WSW and also generally parallel
to the band supporting regenerative back-building. While the
directional convergence is very small 5-10 degrees, this should
help with the width of the band, but still likely to be very
potent in rainfall efficiency given deep layer moisture. With
overshooting tops still blossoming off-shore; an additional 2-3
hours is possible and even as the main core/forcing shifts west
into the Rio Grande Valley, cells with 2-3"/hr rates (hourly
totals of 1.5-2", given transient nature of the cores). As such,
spots of additional 2-4" are possible across the Corpus Christi
corridor.

Eventually, surface moisture convergence is expected to strengthen
along/just west of the coastline and propagation vectors
strengthen slightly to maximize convergence further and further
off-shore after 00z, though blow-off/sheared tops are likely to
translate across the area. Small storm scale interactions may
allow for southwestward drift which would hopefully help to
broaden the rainfall footprint, which would dramatically reduce
the potential for flash flooding. However, trends as they are and
proximity to urban centers already flooding; as such, flash
flooding is likely to continue through 00-01z.

Gallina

ATTN...WFO...BRO...CRP...EWX...

ATTN...RFC...WGRFC...NWC...

LAT...LON 29060050 29009915 28509794 28069726 27789692
27259732 27899888 27489941 27559972 27819996
28490048

= = =
--- SBBSecho 3.14-Linux
* Origin: Ilink: CCO - capitolcityonline.net (454:1/105)